Recent progress in three areas has led to a better understanding of sickle cell pathophysiology: 1)The transport pathways that cause sickle RBC dehydration have been better defined and manipulated in vivo with specific inhibitors; 2) In vivo tracking studies have illuminated time-dependent cellular changes and quantified the behavior of sickle RBC subpopulations in the circulation; 3) Effective therapy to increase HbF with hydroxyurea has resulted in fundamental changes in RBC properties and behavior. However, a number of important issues remain unresolved. It is clear that some sickle cells become dehydrated soon after leaving the bone marrow, but the relative importance of these cells in hemolysis and vasoocclusion remains unknown. The controlling cation depletion pathways for each important RBC subtype, including those defined by age and hemoglobin content, are not defined. This information is required to select appropriate cation transport inhibitors for therapeutic trials. The RBC changes that result from therapy with hydroxyurea are poorly understood, and it is not clear whether the increase in HbF is responsible for all of the clinical effects. In the proposed research, three types of experiments will be performed: 1) Detailed in vivo analyses of RBC subpopulations in density fractions. These experiments will shed light on the extent of dehydration as a function of age and HbF content, and on changes in the behavior of these cellular subtypes with treatment; 2) Investigations of the transport pathways that lead to dehydration of young and mature RBC under oxy and deoxy conditions, and the changes in the activity of these pathways with treatment: 3) In vivo, multiparametric tracking of biotin-labeled, autologous sickle cells to determine the survival and time-dependent hydration change of RBC subtypes, and the changes that occur after effective treatment. The observed red cell behavior will be analyzed in the context of a comprehensive model of sickle cell dehydration and survival. In this model, initial reticulocyte dehydration is dependent on the activity of the KCI cotransport pathway for K efflux and is independent of HbF, while terminal stages of dehydration are sickling dependent, with HbF playing an important role.